1. Field of the Invention
This invention relates generally to the recording and reproduction of PAL television signals. More particularly, it is directed to the conversion of such signals to a form similar to NTSC signals to permit the reduction of cross-talk in the reproduction of such signals recorded in adjacent tracks, even though the relatively low frequency chrominance components of color video signals are recorded for every line interval of each track and even though the tracks are very close together or are even overlapping.
2. The Prior Art
It is well-known to record video signals on magnetic tape or other forms of recording medium by scanning successive parallel tracks on the record medium with one or more transducers energized by the video signals. There has been a constant effort to improve the efficiency of use of the recording medium by packing the tracks as close together as possible. The packing density has always been limited, by, among other things, the fact that, during reproduction of the recorded signals, a reproducing transducer scanning each of the tracks in order could pick up signals or cross-talk from adjacent tracks.
One important step in minimizing cross-talk of low frequency information is disclosed in U.S. patent application Ser. No. 277,815, filed Aug. 3, 1972 and assigned to the assignee of the present case. In some embodiments of that earlier application, the relatively high frequency luminance components were recorded during every line area increment on every track and each line increment area was recorded in side-by-side alignment with line increment areas in the next adjacent tracks on each side. This is known as H alignment and is a technique that has long been used to avoid interference due to picking up horizontal synchronizing signals from one track while picking up signals from the main part of each line interval from the next adjacent track. In accordance with another known technique, the frequency band of the chrominance carrier and components modulated thereon was converted to a relatively low frequency band. Conversion to the low frequency made the chrominance components more susceptible to cross-talk interference from adjacent recorded tracks, but such cross-talk was minimized by not recording the low frequency chrominance components in adjacent line increment areas of adjacent tracks. The chrominance components were recorded intermittently, usually during alternate line intervals, but it was also permissible to record the chrominance components in other intermittent patterns, all of which had the characteristic that chrominance components would not be recorded in side-by-side line increments of two adjacent tracks. If this recording format were visible, the chrominance components would appear to be recorded in a checkerboard-like pattern. Furthermore, the luminance components, as well as chrominance components, could also be recorded intermittently in this same way to permit even further overlapping of adjacent tracks.
In the reproduction of signals recorded with this checkerboard-like pattern, the intermittent components were utilized directly upon reproduction and were also delayed for the length of time necessary to permit them to be used again during the next succeeding interval in which similar information was not recorded. This system reduced the cross-talk interference but at some sacrifice in the quality of the reproduced image.
Another important step in minimizing cross-talk of low frequency signals was disclosed in U.S. patent application Ser. No. 492,330, filed July 26, 1974 and assigned to the assignee of the present case. That application also utilized H alignment and conversion of the frequency of chrominance signal components but recorded the relatively low frequency chrominance signal components in each line increment area of each track. Cross-talk between signals recorded in adjacent tracks was reduced by converting the chrominance sub-carrier differently during each line interval or field interval or both. One embodiment changed the sub-carrier frequency a substantial amount at the beginning of each line interval during alternate fields. Other embodiments changed the frequency only a little or reversed the polarity of the sub-carrier during selected line intervals. Upon reproduction of the signals recorded in a particular track according to any of the proposed formats, the frequency components of the cross-talk signals were frequency-interleaved with frequency components of the desired signals so that the cross-talk components would be intercepted by a comb filter in the playback circuit.
The description of the invention in application Ser. No. 492,330 was based on the recording of NTSC signals. The NTSC standards require that the two chrominance signal components be modulated on sub-carriers of the same frequency but angularly spaced by a fixed 90.degree.. The sub-carriers are then combined into one signal. The sub-carrier frequency is an odd multiple of one-half the line repetition frequency. This makes it possible to cause the chrominance frequency components of potentially interfering line intervals to be offset by an odd multiple of one-fourth of the line repetition frequency and in opposite directions so that these frequency components of potentially interfering signals are spaced apart by an odd multiple of one-half of the line repetition frequency. Such interleaved frequency components are especially suitable for separation by means of a comb filter.
PAL signals are used in a number of countries and are different from the NTSC signals, basically, in that the phase of one of the sub-carriers reverses at the end of each horizontal line interval. Because of the periodic phase reversal of the PAL sub-carrier, the frequency of the latter is offset by only an odd multiple of one-fourth of the line repetition frequency. The line and field repetition frequencies are usually different in NTSC and PAL systems, but those are not fundamental differences.
The periodic reversal of the sub-carrier of one chrominance signal of a PAL signal causes a frequency offset of the harmonic components of one chrominance signal relative to the other. This offset interferes with separation of undesired chrominance signals, which are inadvertently picked up from the next adjacent track, from the desired chrominance signals.
It is one of the objects of this invention to achieve the advantages of periodic reversal or shift of the chrominance carrier for selected line intervals in a video recorder for PAL color television signals to minimize cross-talk.
Another object is to modify the chrominance signals of a PAL signal while converting the frequencies thereof to transform the PAL signals to a form suitable for separating undesired chrominance signals from desired ones by means of a comb filter.
Further objects will be apparent from the following description and the drawings.